This word for potato comes from the digging implement used to uproot them. The word is of unknown origin and was originally used as a term for a short knife or dagger. This sense dates to the 15th century. From Promptorium Parvulorum Sive Clericorum, an Anglo-Latin lexicon from c.1440:

Spudde, cultellus vilis.
(Spudde, an inexpensive little knife)

Over time, spud came to mean a digging tool. From Samuel Pepys Diary of 10 October 1667:

We...begun with a spudd to lift up the ground.

Eventually the word changed in meaning, transferring to the potato from the tool used to dig the tubers up. From Edward Wakefield’s 1845 Adventure in New Zealand:

Pigs and potatoes were respectively represented by ”grunters” and ”spuds.”

An avid reader emailed me with a supposed acronymic origin of spud. The reader rightly was skeptical, but had found the reference in Mario Pei’s 1949 The Story of Language. Pei writes, “the potato, for its part, was in disrepute some centuries ago. Some Englishmen who did not fancy potatoes formed a Society for the Prevention of Unwholesome Diet. The initials of the main words in this title gave rise to spud.” Like all other pre-20th century acronymic origins, this one is false. This just goes to show you, that even language professionals can get taken in sometimes.

(Source: Oxford English Dictionary, 2nd Edition)

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